1. Field of the Invention
The present invention relates to methods of machining explosives, and more specifically, it relates to the use of ultrashort laser pulses for machining explosives.
2. Description of Related Art
Explosives are typically cast into the shape of interest since post machining involves the risk of detonation and/or deflagration and the material is often too soft to be machined. Laser machining (cutting, drilling and sculpting) of high explosives has been attempted several times in the past with a variety of laser sources: Nd:YAG, CO.sub.2, Excimer, Argon-ion, etc. Other than a few studies with the excimer source, all of these approaches were based on localized thermal processing where the purpose of the laser was simply to provide a well localized source of heat to melt or vaporize the material of interest. These approaches met with very limited success since the high temperature associated with the process often resulted in deflagration or worse, detonation of the explosive. A graphical representation of the approximate regimes of laser interaction with explosives is shown in FIG. 1. (On the other hand, the use of these conventional laser sources as a safe means of detonation is attracting increased popularity). Provided in Table I are the ignition temperatures of common explosives (from Ref. 7). Note that even moderate temperature rise of 150-300 degrees (Celsius) above room temperature is enough to ignite most explosives.
TABLE I ______________________________________ Ignition Temperatures (.degree. C.) of selected explosives ______________________________________ Tetrazene 160 RDX 213 Tetryl 180 TNT 240 Nitrocellulose 187 Lead Styphnate 250 Nitroglycerine 188 Lead Azide 350 PETN 205 TATB 359 ______________________________________
The basic interaction in localized thermal processing as is achieved with electron beam or current state of the art lasers is the deposition of energy from the incident beam in the material of interest in the form of heat (lattice vibrations). Absorption of beam energy may differ strongly between different explosives (TATB, TNT, PETN, Composition B, PBX, RDX) dependent upon the optical and thermomechanical properties of the material. The laser energy that is absorbed results in a temperature increase at and near the absorption site. As the temperature increases to the melting or boiling point, material is removed by conventional melting or vaporization. Depending on the pulse duration of the laser, the temperature rise in the irradiated zone may be very fast resulting in thermal ablation and shock. The irradiated zone may be vaporized or simply ablate off due to the fact that the local thermal stress has become larger than the yield strength of the material (thermal shock). In all these cases, where material is removed via a thermal mechanism there is an impact on the material surrounding the site where material has been removed. The surrounding material will have experienced a large temperature excursion or shock often resulting in initiation of a chemical reaction and deflagration.
Another limitation of conventional laser or electron beam (or any thermal based process) in machining high explosives is the vapor produced. This vapor produced by the rapid heating of most common high explosives is extremely toxic and corrosive.